Microstrip antennas comprise a radiator element commonly referred to as a patch. Microstrip patch antennas are highly desirable for aerospace applications because they are lightweight, conformal, and inexpensive since they can be produced using conventional lithographic methods. These microstrip patch antennas are becoming increasingly important because of the proliferation of low Earth orbiting communications and remote sensing satellites that generally demand phased array antenna systems advantageously comprised of microstrip patch antennas. Microstrip patch antennas are known and some of which are described in U.S. Pat. Nos. 5,315,753; 5,448,252; 5,561,435; 5,589,845; 5,694,134; 5,777,581; 5,818,391; 5,838,282; and 5,870,057, all of which are herein incorporated by reference. The patch geometry can be square, rectangular, a disk or an annular ring. A major drawback of microstrip antennas is their inherently narrow instantaneous bandwidth, typically 1% or so. Intuitively obvious approaches to enhance bandwidth, such as the use of extremely low permittivity substrates or thick substrates are typically met with an undesirable increase in antenna size or the generation of surface waves that degrade the efficiency of the antennas.
Several approaches are known to increase patch antenna bandwidth. For example, stacked patches have been used to generate dual resonant frequencies. In this approach, a bottom patch is covered with a dielectric layer that serves as the substrate for a top patch. The bottom patch serves as a ground plane for the top patch. Bancroft in a technical article "Accurate Design of Dual-Band Antennas," Microwaves & RF, September, 1988, pp. 113-118, herein incorporated by reference, describes such a bottom patch covered with a dielectric layer and operating at 9 and 11 GHz, a difference of about 20%. Another approach is to use varactor diodes to modify the resonant frequency and is described in a technical article "Active Patch Antenna Element with Diode Tuning," of P. Haskins, P. Hall, and J. Dahele, Electronics Letters, Vol. 27, No. 20, September, 1991, pp. 1846-1847, which is herein incorporated by reference. Haskins et al integrated a diode with a multilayer patch and obtained a 4% tuning range. Navarro and Chang in a technical article "Broadband Electronically Tunable IC Active Radiating Elements and Power Combiners," Microwave Journal, October, 1992, pp. 87-101, herein incorporated by reference, integrated a varactor with a notch antenna and achieved tuning from 8.9 to 10.2 GHz, a range of about 14%. Kiely, Washington, and Bernhard in a technical article "Design and Development of Smart Microstrip Patch Antennas," Journal of Smart Materials and Structures, Vol. 7. pp. 792-800, 1998, herein incorporated by reference, arranged a patch above a parasitic element and varied the separation therebetween by using piezoelectric actuators to shift the frequency. Rainville and Harackiewicz in a technical article IEEE Micro Guided Wave Lett., Vol. 12, no. 2, pp. 483-485, 1992, herein incorporated by reference, describe a patch fabricated on a ferrite film. The application of an in-plane magnetic field onto this ferrite film advantageously tuned the resonant frequency of a cross-polarized field, but not the co-polarized field. The tuning range was 5.86 to 6.03 GHz, about 3%. Although each of these efforts further advanced the art, it is desired that further improvement be made to further increase patch antenna bandwidth so as to enhance their application to both the military and commercial endeavors. Commercial and military applications include low cost tracking terminals to advantageously complement the forthcoming wideband low Earth orbiting satellite constellations and stealthy communications and radar systems.